Study On The Mineralogy Of Saidikulamu Nephrite,Yutian

Saidikulamu nephrite deposits which adjacent to the famous Alamas nephrite deposits,is located in Yutian county,Xinjiang province.The two nephrite deposits have the similar regional geological background.Saidikulamu nephrite deposits located in Kangxiwa-Kangbashen tectonic line,the hyperplasia of west kunlun orogenic belt.Saidikulamu nephrite deposits mainly produce Green nephrite,few white nephrite and Qinghua(with graphite).In order to identify the mineral composition,structure,paragenetic relationship of associated minerals and the genesis of Saidikulamu nephrite,9 typical samples nephrite of Saidikulamu nephrite deposits are selected,using conventional gemological testing,polarizing microscope,infrared spectrum,electron probe and X-ray powder diffraction experiment to analyse.The main mineral of Saidikulamu nephrite is nephrite(more than 98%),the accessory minerals are epidote,apatite,calcite,diopside,graphite,hematite,allanite,zircon,chlorite,etc.One of the most common accessory mineral is epidote,often exist as the "white spot" in the green Saidikulamu nephrite.The typical structure of Saidikulamu nephrite is fibrous crystalloblastic structure.In addition,the brush structure,sheaf texture,the metasomatic phenocryst structure.There are also exsit the metasomatic pseudomorphic texture,near directional structure,secondary filling structure.The average degree of crystallinity of Saidikulamu nephrite is 0.890,lower than those of other Chinese origin.The crystallinity of nephrite may have great relation to its strong oily luster.Under the polarizing microscope,there are evidnce of stress,for example,the almost orientationed fibrous tremolite,roughly circular appearance of rolling-like mineral grains between both sides of the almost orientationed fibrous tremolite,a set of parallel-like fractures along the roughly directional fibrous tremolite,etc.It is showed that Saidikulamu nephrite may be formed under the continuous strong ductile shearing force.The formation of Saidikulamu nephrite deposits is divided into three stages:(1)Regional metamorphism of dolomitic marble: magnesian marble formed from dolomite and calcite by regional metamorphism.(2)Contact metasomatism-hydrothermal metasomatism between dolomitic marble and granite diorite intrusive mass: generated early coarse grained tremolite and late fibrous amphibole,the latter is the main body of nephrite.(3)Secondary stage: after the formation of Saidikulamu nephrite,iron oxide is disseminated along the fracture of nephrite.The zircon picked from itrusive rock in Saidikulamu nephrite mining area using for SHRIMP U-Pb dating experiments,two groups of age data are measured: 454 Ma and 489 Ma.It can be deduced that Saidikulamu nephrite deposits had experienced two magmatic acitivities.Saidikulamu nephrite deposits may be formed in the ordovician,not late variscan.